Modern vehicles use three-way catalysts (TWC) for exhaust after-treatment of gasoline engines. With tightening government regulations on automobile emissions, feedback control is used to adequately regulate the engine air-to-fuel ratio (AFR). Some vehicles have a universal exhaust gas oxygen (UEGO) sensor upstream of the TWC and a heated exhaust gas oxygen (HEGO) sensor downstream of the TWC to control the AFR near stoichiometry. Feedback AFR control in cylinders is achieved by regulating the AFR to a desired AFR around stoichiometry, which in turn is fine-tuned based on the deviation of a HEGO voltage from a pre-determined HEGO-voltage set-point.
However, the physical geometry and arrangement of engine cylinders creates a non-uniform, zoned exhaust flow condition in the exhaust system that makes in cylinder AFR difficult to determine. Various conditions, such as an AFR imbalance between cylinders, may exacerbate this non-uniform, zoned exhaust flow condition so that the UEGO sensor may not equally detect all of the cylinders. An AFR imbalance between cylinders occurs when the AFR in one or more cylinders is different than the AFR in other cylinders due to a cylinder-specific condition, such as an intake manifold leak at a particular cylinder, a clogged fuel injector, an individual cylinder exhaust gas recirculation runner imbalance, or a fuel-flow delivery issue. Due to the zoned exhaust flow, a cylinder with an air-fuel imbalance may only be detected if the cylinder has relatively large imbalance. Thus, smaller imbalances may go undetected, leading to significant feedgas emissions such as carbon monoxide (CO) or the oxides of nitrogen (NOx) passing directly to the tailpipe, as the biased air-fuel mixture is fed directly to the catalyst, overwhelming the oxygen-storage buffer that allows for short deviations from stoichiometry.
The inventors herein have recognized the above issues and have devised various approaches to solve them. In particular, systems and methods for providing the technical result of identifying and mitigating air-fuel imbalance conditions specific to an engine cylinder are provided. In one example, a method comprises adjusting engine operation based on an indication of cylinder air-fuel imbalance, the cylinder air-fuel imbalance detected based on output from a second sensor and a plurality of individual cylinder weighting factors, the second sensor located in an exhaust system at a location downstream of a first sensor located in the exhaust system.
In this way, cylinder air-fuel imbalance may be detected based on the composition of the exhaust gas measured by the second exhaust gas sensor. The exhaust gas that passes by the second exhaust gas sensor is a relatively homogenous mix of the exhaust streams from all cylinders, and thus each cylinder's air-fuel ratio may be equally detected. In order to determine the air-fuel ratio of each cylinder while only measuring a mix of exhaust gas, rather than individual slugs that correspond to each individual cylinder, a plurality of individual cylinder weighting factors are applied to the output from the second exhaust gas sensor. The plurality of individual cylinder weighting factors may reflect each cylinder's contribution to the air-fuel ratio detected by the first exhaust gas sensor, over a plurality of engine operating conditions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.